The oxidation of organic materials to carbon dioxide and water is a process known almost since the beginning of time and often used to dispose of waste materials and/or generate useful energy such as steam for heating, power generation and in some cases, for desalination of seawater. In conventional generation of energy in the form of steam often organic fuels are oxidized rapidly in combustion to produce heat which is then transferred through a heat exchanger to a fluid such as water. There are inherent losses in this conventional system. For example, often 10 to 15% of the heating value of the fuel is necessarily lost in the exhaust stack of conventional boilers. In addition, the heat exchangers necessarily add to cost and expense and are often of relatively large size. Heat transfer through surfaces of heat exchangers sometimes causes problems and often requires the use of specialized materials when high temperatures are involved. Hot spots due to salt deposition on boiler tubes can cause expensive down-time due to rupture of tube walls. On the flame or hot gas sides of the tubes, ash or other deposits often impede heat flow and reduce heat transfer efficiency.
In one known process for treating waste organic materials, i.e., the wet air oxidation process, an organic feed and oxidizing agent are pressurized to reaction conditions of from about 1500 to 2500 psia, heated to operating temperature and fed to a reactor for residence times of 0.5 to 1 hour. This process is known to be effective for removing 70-95% of the initial organic material. This system is effective but has certain disadvantages. It is often costly in that large size equipment is necessary and inefficient recovery of the heat of combustion is obtained. Often the solubility of oxygen or air in water is below the level required for complete oxidation of the organic materials. Thus, a two-phase water-gas mixture is often used in the reactor, necessitating provisions for agitation in the reactor so as to avoid excessive mass transfer resistance between the phases. This tends to make the reactor somewhat complicated and more expensive than would otherwise be necessary. Often volatile organics such as acetic acid remain after complete processing. Long residence times are needed and the reactions are often not adiabatic which results in loss of part of the heat of combustion to the environment. When energy is recovered in the form of steam, the temperature of the steam produced is below that of the reactor effluent, which is usually below 300.degree. C. and typically in the range of 250.degree. C. Thus, the heat recovered is of a low to moderate value and significantly below that required for generating electrical power in modern steam cycle power plants.
It has been suggested to recover heat energy from supercritical water effluent and the technology for doing this is highly developed. Electrical utilities since the 50's have used supercritical water power cycles to generate power from fossil fuels.
The known literature describes production of supercritical water by burning fossil fuels followed by the use of equipment for recovering heat from the supercritical water and turning the heat into power. Thus, such recovery systems are known. Rankine cycle type equipment can be used to recover useful energy from water at supercritical conditions and temperatures above 450.degree. C. as known in the art.
It has been suggested that toxic organic materials can be reformed at the supercritical conditions of water to harmless lower molecular weight materials by breakdown of organic chains and the like whereby the resulting non-toxic materials can be disposed.